Alkylene glycols, such as ethylene glycol and propylene glycol, are widely used as raw materials in the production of polyesters, polyethers, antifreeze, solution surfactants, and as solvents and base materials in the production of polyethylene terephthalates (e.g. for fibers or bottles). Commercial processes for the preparation of alkylene glycols typically involve the liquid phase hydration of the corresponding epoxide in the presence of a large molar excess of water (see, e.g., Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 11, Third Edition, page 929 (1980)).
Ethylene glycol is commonly produced by the noncatalytic reaction of ethylene oxide and water. The reactions are run adiabatically, and the heat of reaction is absorbed by the reacting fluids which respond with an increase in temperature. The reaction temperature is typically 120.degree. C. at the inlet to the reactor and often exceeds 180.degree. C. at the exit point.
High temperatures are desirable in the preparation of ethylene glycol because the rate of reaction is maximized and selectivity is unaffected by high temperature. An added advantage of high temperature operation is that it reduces the need to supply external sources of heat to downstream purification equipment for the separation and recovery of unreacted water from the ethylene glycol product.
High ratios of water to ethylene oxide are typically fed to the commercial reactors to favor the production of mono-ethylene glycol, which is capable of also reacting with ethylene oxide to form diethylene glycol. Additionally, the diethylene glycol can react with ethylene oxide to form triethylene glycol, and so forth.
Formation of higher glycols is viewed as commercially unattractive, since the production of these higher glycols consumes valuable ethylene oxide, and markets for use of higher glycols are limited. The use of excessive quantities of water to favor mono-ethylene glycol add to the cost of manufacture because the excess water must be removed with energy through capital intensive evaporation and distillation process steps.
Catalytic systems have recently been studied for the purpose of selectively hydrolyzing epoxides, although commercialization has been an elusive goal. For example, JP 57-139026 teaches a catalyzed process utilizing anion exchange resins in the chloride form and carbon dioxide resulting in superior selectivity over comparable non-catalyzed or thermal processes. One drawback to the process taught in JP is the formation of ethylene carbonate, separation of which is difficult and expensive.
Examples of catalytic processes are also taught in RU 2001901 and RU 2002726. Therein are taught processes for converting a catalyst to the bicarbonate form before the catalytic reaction, and reducing the concentration of carbon dioxide to as low as 0.01 percent by weight in order to allow the catalyst to be more selective toward monoethylene glycol.
U.S. Pat. No. 5,488,184 (the '184 Patent) also teaches a catalytic process wherein carbon dioxide is reduced or eliminated from the reaction mixture in order to enable higher reaction rates. The '184 Patent teaches that, for the bisulfite form of the catalyst, addition of carbon dioxide is beneficial to the reaction selectivity, but that for other anion forms of the catalyst, including the bicarbonate and formate forms, addition of carbon dioxide is detrimental to selectivity as well as the reaction kinetics for the bicarbonate form. The '184 Patent thus teaches that the concentration of carbon dioxide be kept below 0.1 wt %. The '184 Patent also teaches using relatively low reaction temperatures of around 80.degree. C. Such low reaction temperatures require external cooling to maintain.
PCT publications WO 99/31034 and WO 99/31033 also teach catalytic processes at relatively low reaction temperatures. Such references teach advantageously using a specific reactor design and adjusting the pH, respectively, to prolong the catalyst lifetime and minimize catalyst swelling.
The aforementioned references are limited by low reaction temperature, due primarily to the fact that anion exchange resins in the bicarbonate form, if exposed to high temperatures, typically deactivate quickly, as quickly as a few days when temperatures exceed 120.degree. C. Because the hydrolysis reaction is exothermic, even higher reaction temperatures would be desired to permit maximum temperature rise without cooling.
Commercialization of catalytic processes require that the catalyst be stable for an extended period of time. Otherwise, plant shut-downs to remove the catalyst from the reactor result in added expense and significant economic disadvantage.
Thus a catalytic system is desired that provides a combination of long catalyst life with minimal physical and chemical changes while operating at high temperature with efficient use of energy.